Page 1 of 1

### PV=nRT

Posted: Fri Jan 19, 2018 11:15 pm
Can somebody explain the significance of PV=nRT and the role it plays in the chapter. How does it relate to enthalpies? Does it have some relationship to DeltaU=q+w, and can you explain it?

### Re: PV=nRT

Posted: Fri Jan 19, 2018 11:22 pm
Hi. I think we use PV=nRT in this chapter to calculate enthalpy. In the example in class today, it says that change in enthalpy = change in internal energy + P (change in V). If we do not know the change in volume of the system and it is not given, we can use the change in moles of gas to calculate the change in enthalpy. The equation would then become change in enthalpy = change in internal energy to (change in n)RT.

Also under constant pressure, work equls -P(change in V), so you could also substitute -(change in n)RT for work as well.

### Re: PV=nRT

Posted: Fri Jan 19, 2018 11:24 pm
As we know, change in internal energy equals w+q and w=-q*∆V. For gases, when it's under constant pressure and there is a net change in the number of moles, there will be a change in the volume. According to PV=nRT, P∆V=∆nRT. Therefore, when a reaction under constant pressure and has a net change in the number of moles, the work done by the system can be calculated by using ∆nRT.
Hope it helps.

### Re: PV=nRT

Posted: Sat Jan 20, 2018 12:06 am
If the system expands or is compressed is there still a change in the net number of moles? If not, would the equation P∆V=∆nRT still hold true because the net number of moles aren't changing?

### Re: PV=nRT

Posted: Sat Jan 20, 2018 11:39 pm
If the system expands or is compressed, there can only be a change in the number of moles if the system is open.